(a) Field of the Invention
This invention relates to a method for coating on a substrate an aqueous paint comprising a dispersion of a fluoroethylene homopolymer or copolymer resin alone or together with an inorganic filler in an aqueous medium, a method for coating on the above-mentioned coating layer (used as a primer layer) a fluoroethylene resin, and a structure of the resulting coating film.
(B) Description of the Prior Art
Fluoroethylene resins, i.e., homopolymers of a fluoroethylene or copolymers comprising a major amount of a fluoroethylene such as tetrafluoroethylene, trifluoroethylene, vinylidene fluoride and trifluoromonochloroethylene have excellent chemical resistances, as well-known, to most acids and alkalis, halogen gases, oxidative chemicals, organic solvents such as halohydrocarbons, and the like. The resins are mechanically strong in that they have large tensile strength and impact strength, and excellent creep resistance and abrasion resistance. Furthermore, they are thermally stable in that fusion temperature is in the range of 150.degree.-320.degree. C. and decomposition temperature is in the range of 200.degree.-340.degree. C. In other words, it is expected from these properties that fluoroethylene resins are very preferable materials for the anti-corrosive coating onto chemical process apparatus.
As to the methods for coating with fluoroethylene resins, there are known the method for coating with the resin dispersed in organic solvents such as an organosol, the method for coating with the aqueous latexes and the method for powder coating with the dry resin powder.
The organosol method which employs organic solvents has the defects in that workers may be exposed to harmful vapors of the solvents upon coating operations and fire is apt to occur. The aqueous latex becomes unstable at a higher concentration of the resins. It is required to use a large amount of emulsifiers in order to increase emulsion stability of the latex. The problems reside in that a general emulsifier is apt to decompose at a temperature for coating with fluoroethylene resins and a perfluororesin acid type emulsifier having good thermal stability is very expensive. A latex of lower concentration is generally employed. When such a dilute latex is applied onto a perpendicular wall, a very thin coating film is obtained by each coating operation since such latex is of low viscosity and easily flows down the wall. For example, in order to obtain a coating film as thick as 1 mm, the operation of coating--drying--baking must be repeated for several tens of times.
The method for powder coating is most suitable for obtaining relatively thick coating film by one coating operation. The powder coating method has also some defects given below.
The apparatus which is to be used at a relatively low temperature and a dry state may be coated with fluoroethylene resins alone. But, the chemical process apparatus which is to be used under corrosive circumstances of high temperature and high humidity suffers from the following troubles, even if the coated resin film does not undergo corrosion or swelling.
One of the troubles is that the coating film on a substrate having concave sufaces peels off at its concave surfaces and the peeling spreads outside. Another trouble is that a lot of blisters are produced and the coating peels off from the surface of the substrate as the blisters grow. These defects cause a decrease in the durable life of the coated apparatus. Various efforts to depress the formation of blisters have been made. For example, a primer coating layer has been placed between an anti-corrosive coating layer and a substrate. In this case, the primer coating layer should have a good bonding property to the anti-corrosive surface coating layer. In the primer coating layer is generally employed the resin of the same kind as the surface coating resin or the resin having good compatibility therewith. The powder coating method may sometimes be applied to a primer coating layer. It is generally difficult to form a uniform coating film on a curved or uneven surface with the powder coating method. Therefore, the method is generally not suitable for forming a relatively thin primer coating.
Such peeling off is apt to occur when the coating film is thicker and when the substrate has a curved surface of a large curvature. The coefficient of thermal expansion of thermoplastic resins is generally about 10 times as large as that of metals. Therefore, a large strain owing to the difference in thermal expansion takes place at the bonding surface between the metal substrate and the coating resin layer when temperature changes. Thus, the coating resin layer peels off. In this case, the thicker the coating film is, the larger the strain on the bonding surface. The strain is concentrated into the place having a large curvature. Accordingly, a relatively thick coating film is formed by the powder coating method, and peeling off is most liable to occur.
The intensive efforts to enhance bonding strength between the resin and a substrate have hitherto been made for the purpose of eliminating formation of blisters. But, it seems that such enhancement of bonding strength scarcely serves to prevent blisters. In other words, peeling between a substrate and a coating film due to formation of blisters often depends on cracks in the resin film itself which takes place in the vicinity of the bending portion therebetween. Therefore, the enhancement of bonding strength by itself would not solve the problems.
The present inventors developed the following hypothesis. Small cracks are at first produced at the bonding surface between a substrate and a coating layer or at the resin layer in the vicinity thereof owing to the difference in thermal expansion between a fluorinated thermoplastic resin and a metal substrate. Water or a chemical liquid may more or less permeate into a synthetic resin. Since a very small amount of a chemical liquid or water can permeate into a fluoroethylene thermoplastic resin having no pinholes, the chemical liquid penetrates into the pores in the resin layer for a long period of time, which were formed by cracks in the resin layer. The chemical liquid which has penetrated into the pores expands the pores due to liquid expansion on elevation of its vapor pressure at an elevated temperature. When temperature falls, the resulting reduced volume of pressure facilitates penetration of the chemical liquid into the expanded pores. When the vaporization and condensation of the chemical liquid are repeated, there occurs a very large difference in pressure. Owing to the repeated vaporization and condensation, blisters may grow for a long period of time to result in peeling of a coating, or blistered portions may produce holes owing to repeated strains at the blistered portions.
From the above mentioned points of view, sufficient voids were given to the primer layer of a coating film. Thus, the difference in thermal expansion can be absorbed by the voids, and air contained in the voids can moderate the elevation of vapor pressure caused by the penetrated chemical liquid. Formation of blisters can be regulated by these synergistic effects. It has some effects on the regulation of blisters to incorporate an inorganic powder having a small coefficient of thermal expansion into a primer layer and decrease the average coefficient of thermal expansion of the mixed composition comprising the inorganic powder and resin. In this case, the bonding strength to a substrate is naturally decreased when the porous primer layer is employed. It is considered, however, that the coating film is substantially prevented from peeling by regulating formation of blisters.